Image sensors having high dynamic range imaging pixels

ABSTRACT

A high dynamic range imaging pixel may include a photodiode that generates charge in response to incident light. When the generated charge exceeds a first charge level, the charge may overflow through a first transistor to a first storage capacitor. When the generated charge exceeds a second charge level that is higher than the first charge level, the charge may overflow through a second transistor. The charge that overflows through the second transistor may alternately be coupled to a voltage supply and drained or transferred to a second storage capacitor for subsequent readout. Diverting more overflow charge to the voltage supply may increase the dynamic range of the pixel. The amount of charge diverted to the voltage supply may therefore be updated to control the dynamic range of the imaging pixel.

BACKGROUND

This relates generally to imaging devices, and more particularly, toimaging devices having high dynamic range imaging pixels.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. In a typicalarrangement, an image sensor includes an array of image pixels arrangedin pixel rows and pixel columns. Circuitry may be coupled to each pixelcolumn for reading out image signals from the image pixels.

Typical image pixels contain a photodiode for generating charge inresponse to incident light. Image pixels may also include a chargestorage region for storing charge that is generated in the photodiode.Image sensors can operate using a global shutter or a rolling shutterscheme. In a global shutter, every pixel in the image sensor maysimultaneously capture an image, whereas in a rolling shutter each rowof pixels may sequentially capture an image.

Some conventional image sensors may be able to operate in a high dynamicrange (HDR) mode. HDR operation may be accomplished in image sensors byassigning alternate rows of pixels different integration times. However,conventional image sensors may sometimes experience lower than desiredresolution, lower than desired sensitivity, higher than desired noiselevels, and lower than desired quantum efficiency.

It would therefore be desirable to be able to provide improved highdynamic range operation in image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having an imagesensor in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals in an image sensor inaccordance with an embodiment.

FIG. 3 is a circuit diagram of an illustrative imaging pixel thatincludes an overflow capacitor in accordance with an embodiment.

FIG. 4 is a circuit diagram of an illustrative imaging pixel thatincludes a first overflow capacitor and a charge directing structurethat intermittently directs charge to a second overflow capacitor inaccordance with an embodiment.

FIG. 5 is a timing diagram showing operation of an illustrative imagingpixel such as the pixel of FIG. 4 when more overflow charge is disposedof then integrated in accordance with an embodiment.

FIG. 6 is a timing diagram showing operation of an illustrative imagingpixel such as the pixel of FIG. 4 when more overflow charge isintegrated than disposed of in accordance with an embodiment.

FIG. 7 is a flowchart showing illustrative method steps for operating animaging pixel with two overflow paths such as the pixel of FIG. 4 inaccordance with an embodiment.

FIG. 8 is a circuit diagram of an illustrative imaging pixel thatincludes two overflow paths, that is split between two substrates, andthat includes a conductive interconnect structure between the twosubstrates in accordance with an embodiment.

FIG. 9 is a circuit diagram of illustrative imaging pixels with twooverflow paths that share a floating diffusion region in accordance withan embodiment.

FIG. 10 is a diagram of an illustrative pixel array showing how everytwo pixels may share a floating diffusion region in accordance with anembodiment.

FIG. 11 is a diagram of an illustrative pixel array showing how everyfour pixels may share a floating diffusion region in accordance with anembodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors. It will berecognized by one skilled in the art that the present exemplaryembodiments may be practiced without some or all of these specificdetails. In other instances, well-known operations have not beendescribed in detail in order not to unnecessarily obscure the presentembodiments.

Electronic devices such as digital cameras, computers, cellulartelephones, and other electronic devices may include image sensors thatgather incoming light to capture an image. The image sensors may includearrays of pixels. The pixels in the image sensors may includephotosensitive elements such as photodiodes that convert the incominglight into image signals. Image sensors may have any number of pixels(e.g., hundreds or thousands or more). A typical image sensor may, forexample, have hundreds of thousands or millions of pixels (e.g.,megapixels). Image sensors may include control circuitry such ascircuitry for operating the pixels and readout circuitry for reading outimage signals corresponding to the electric charge generated by thephotosensitive elements.

FIG. 1 is a diagram of an illustrative imaging and response systemincluding an imaging system that uses an image sensor to capture images.System 100 of FIG. 1 may be an electronic device such as a camera, acellular telephone, a video camera, or other electronic device thatcaptures digital image data, may be a vehicle safety system (e.g., anactive braking system or other vehicle safety system), or may be asurveillance system. As shown in FIG. 1, system 100 may include animaging system such as imaging system 10 and host subsystems such ashost subsystem 20. Imaging system 10 may include camera module 12.Camera module 12 may include one or more image sensors 14 and one ormore lenses.

Each image sensor in camera module 12 may be identical or there may bedifferent types of image sensors in a given image sensor arrayintegrated circuit. During image capture operations, each lens may focuslight onto an associated image sensor 14. Image sensor 14 may includephotosensitive elements (i.e., pixels) that convert the light intodigital data. Image sensors may have any number of pixels (e.g.,hundreds, thousands, millions, or more). A typical image sensor may, forexample, have millions of pixels (e.g., megapixels). As examples, imagesensor 14 may include bias circuitry (e.g., source follower loadcircuits), sample and hold circuitry, correlated double sampling (CDS)circuitry, amplifier circuitry, analog-to-digital converter circuitry,data output circuitry, memory (e.g., buffer circuitry), addresscircuitry, etc.

Still and video image data from camera sensor 14 may be provided toimage processing and data formatting circuitry 16 via path 28. Imageprocessing and data formatting circuitry 16 may be used to perform imageprocessing functions such as data formatting, adjusting white balanceand exposure, implementing video image stabilization, face detection,etc. Image processing and data formatting circuitry 16 may also be usedto compress raw camera image files if desired (e.g., to JointPhotographic Experts Group or JPEG format). In a typical arrangement,which is sometimes referred to as a system on chip (SOC) arrangement,camera sensor 14 and image processing and data formatting circuitry 16are implemented on a common semiconductor substrate (e.g., a commonsilicon image sensor integrated circuit die). If desired, camera sensor14 and image processing circuitry 16 may be formed on separatesemiconductor substrates. For example, camera sensor 14 and imageprocessing circuitry 16 may be formed on separate substrates that havebeen stacked.

Imaging system 10 (e.g., image processing and data formatting circuitry16) may convey acquired image data to host subsystem 20 over path 18.Host subsystem 20 may include processing software for detecting objectsin images, detecting motion of objects between image frames, determiningdistances to objects in images, filtering or otherwise processing imagesprovided by imaging system 10.

If desired, system 100 may provide a user with numerous high-levelfunctions. In a computer or advanced cellular telephone, for example, auser may be provided with the ability to run user applications. Toimplement these functions, host subsystem 20 of system 100 may haveinput-output devices 22 such as keypads, input-output ports, joysticks,and displays and storage and processing circuitry 24. Storage andprocessing circuitry 24 may include volatile and nonvolatile memory(e.g., random-access memory, flash memory, hard drives, solid-statedrives, etc.). Storage and processing circuitry 24 may also includemicroprocessors, microcontrollers, digital signal processors,application specific integrated circuits, etc.

An example of an arrangement for camera module 12 of FIG. 1 is shown inFIG. 2. As shown in FIG. 2, camera module 12 includes image sensor 14and control and processing circuitry 44. Control and processingcircuitry 44 may correspond to image processing and data formattingcircuitry 16 in FIG. 1. Image sensor 14 may include a pixel array suchas array 32 of pixels 34 (sometimes referred to herein as image sensorpixels, imaging pixels, or image pixels 34) and may also include controlcircuitry 40 and 42. Control and processing circuitry 44 may be coupledto row control circuitry 40 and may be coupled to column control andreadout circuitry 42 via data path 26. Row control circuitry may receiverow addresses from control and processing circuitry 44 and may supplycorresponding row control signals to image pixels 34 over control paths36 (e.g., dual conversion gain control signals, pixel reset controlsignals, charge transfer control signals, blooming control signals, rowselect control signals, or any other desired pixel control signals).Column control and readout circuitry 42 may be coupled to the columns ofpixel array 32 via one or more conductive lines such as column lines 38.Column lines 38 may be coupled to each column of image pixels 34 inimage pixel array 32 (e.g., each column of pixels may be coupled to acorresponding column line 38). Column lines 38 may be used for readingout image signals from image pixels 34 and for supplying bias signals(e.g., bias currents or bias voltages) to image pixels 34. During imagepixel readout operations, a pixel row in image pixel array 32 may beselected using row control circuitry 40 and image data associated withimage pixels 34 of that pixel row may be read out by column control andreadout circuitry 42 on column lines 38.

Column control and readout circuitry 42 may include column circuitrysuch as column amplifiers for amplifying signals read out from array 32,sample and hold circuitry for sampling and storing signals read out fromarray 32, analog-to-digital converter circuits for converting read outanalog signals to corresponding digital signals, and column memory forstoring the read out signals and any other desired data. Column controland readout circuitry 42 may output digital pixel values to control andprocessing circuitry 44 over line 26.

Array 32 may have any number of rows and columns. In general, the sizeof array 32 and the number of rows and columns in array 32 will dependon the particular implementation of image sensor 14. While rows andcolumns are generally described herein as being horizontal and vertical,respectively, rows and columns may refer to any grid-like structure(e.g., features described herein as rows may be arranged vertically andfeatures described herein as columns may be arranged horizontally).

Pixel array 32 may be provided with a color filter array having multiplecolor filter elements which allows a single image sensor to sample lightof different colors. As an example, image sensor pixels such as theimage pixels in array 32 may be provided with a color filter array whichallows a single image sensor to sample red, green, and blue (RGB) lightusing corresponding red, green, and blue image sensor pixels arranged ina Bayer mosaic pattern. The Bayer mosaic pattern consists of a repeatingunit cell of two-by-two image pixels, with two green image pixelsdiagonally opposite one another and adjacent to a red image pixeldiagonally opposite to a blue image pixel. In another suitable example,the green pixels in a Bayer pattern are replaced by broadband imagepixels having broadband color filter elements (e.g., clear color filterelements, yellow color filter elements, etc.). These examples are merelyillustrative and, in general, color filter elements of any desired colorand in any desired pattern may be formed over any desired number ofimage pixels 34.

If desired, array 32 may be part of a stacked-die arrangement in whichpixels 34 of array 32 are split between two or more stacked substrates.In such an arrangement, each of the pixels 34 in the array 32 may besplit between the two dies at any desired node within the pixel. As anexample, a node such as the floating diffusion node may be formed acrosstwo dies. Pixel circuitry that includes the photodiode and the circuitrycoupled between the photodiode and the desired node (such as thefloating diffusion node, in the present example) may be formed on afirst die, and the remaining pixel circuitry may be formed on a seconddie. The desired node may be formed on (i.e., as a part of) a couplingstructure (such as a conductive pad, a micro-pad, a conductiveinterconnect structure, or a conductive via) that connects the two dies.Before the two dies are bonded, the coupling structure may have a firstportion on the first die and may have a second portion on the seconddie. The first die and the second die may be bonded to each other suchthat first portion of the coupling structure and the second portion ofthe coupling structure are bonded together and are electrically coupled.If desired, the first and second portions of the coupling structure maybe compression bonded to each other. However, this is merelyillustrative. If desired, the first and second portions of the couplingstructures formed on the respective first and second dies may be bondedtogether using any metal-to-metal bonding technique, such as solderingor welding.

As mentioned above, the desired node in the pixel circuit that is splitacross the two dies may be a floating diffusion node. Alternatively, thedesired node in the pixel circuit that is split across the two dies maybe the node between a floating diffusion region and the gate of a sourcefollower transistor (i.e., the floating diffusion node may be formed onthe first die on which the photodiode is formed, while the couplingstructure may connect the floating diffusion node to the source followertransistor on the second die), the node between a floating diffusionregion and a source-drain node of a transfer transistor (i.e., thefloating diffusion node may be formed on the second die on which thephotodiode is not located), the node between a source-drain node of asource follower transistor and a row select transistor, or any otherdesired node of the pixel circuit.

In general, array 32, row control circuitry 40, column control andreadout circuitry 42, and control and processing circuitry 44 may besplit between two or more stacked substrates. In one example, array 32may be formed in a first substrate and row control circuitry 40, columncontrol and readout circuitry 42, and control and processing circuitry44 may be formed in a second substrate. In another example, array 32 maybe split between first and second substrates (using one of the pixelsplitting schemes described above) and row control circuitry 40, columncontrol and readout circuitry 42, and control and processing circuitry44 may be formed in a third substrate.

To increase high dynamic range in imaging pixels, imaging pixels may beformed with an overflow capacitor. The photodiode for the imaging pixelgenerates charge. After the charge exceeds a certain level, the excesscharge may overflow the photodiode into a storage capacitor. The chargefrom both the storage capacitor and the photodiode may be read out afterthe integration time is complete. This effectively increases the dynamicrange of the pixel. The storage capacitor in these types of pixels maybe referred to as an overflow capacitor.

FIG. 3 is a circuit diagram of an imaging pixel having a photosensitiveelement and a storage capacitor. As shown in FIG. 3, image pixel 34includes photosensitive element 102 (e.g., a photodiode). Photosensitiveelement 102 has a first terminal that is coupled to ground. The secondterminal of photosensitive element 102 is coupled to transfer transistor104 and transistor 105. Transfer transistor 104 is coupled to floatingdiffusion (FD) region 118. Transistor 105 (sometimes referred to asthreshold transistor 105) is coupled between photodiode 102 and storagecapacitor 110. A reset transistor 106 may be coupled between floatingdiffusion region 118 and voltage supply 124. Voltage supply 124 mayprovide a voltage VDD. Floating diffusion region 118 may be a dopedsemiconductor region (e.g., a region in a silicon substrate that isdoped by ion implantation, impurity diffusion, or other doping process).Floating diffusion 118 has an associated capacitance. Gain selecttransistor 108 has a first terminal coupled to floating diffusion region118 and a second terminal coupled to storage capacitor 110 (Cx). Dualconversion gain capacitor 110 may have a first plate 110-1 (sometimesreferred to as an upper plate or top plate) coupled to the secondterminal of the gain select transistor. Dual conversion gain capacitor110 may have a second plate 110-2 (sometimes referred to as a lowerplate or bottom plate) that is coupled to voltage supply 126. Voltagesupply may provide a voltage Vxx.

Source follower transistor 112 has a gate terminal coupled to floatingdiffusion region 118 and a first terminal of reset transistor 106.Source follower transistor 112 also has a first source-drain terminalcoupled to voltage supply 120. Voltage supply 120 may provide a powersupply voltage VDD. The power supply voltage at power supplies 120 and124 may be the same or may be different. In this application, eachtransistor is illustrated as having three terminals: a source, a drain,and a gate. The source and drain terminals of each transistor may bechanged depending on how the transistors are biased and the type oftransistor used. For the sake of simplicity, the source and drainterminals are referred to herein as source-drain terminals or simplyterminals. A second source-drain terminal of source follower transistor112 is coupled to column output line 116 through row select transistor114. The source follower transistor, row select transistor, and columnoutput line may sometimes collectively be referred to as a readoutcircuit or as readout circuitry.

A gate terminal of transfer transistor 104 receives control signal TX. Agate terminal of threshold transistor 105 receives control signal TH1. Agate terminal of gain select transistor 108 receives control signal DCG.A gate terminal of reset transistor 106 receives control signal RST. Agate terminal of row select transistor 114 receives control signal RS.Control signals TX, DCG, RST, RS, and TH1 may be provided by row controlcircuitry (e.g., row control circuitry 40 in FIG. 2) over control paths(e.g., control paths 36 in FIG. 2).

Gain select transistor 108 and dual conversion gain capacitor 110 may beused by pixel 34 to implement a dual conversion gain mode. Inparticular, pixel 34 may be operable in a high conversion gain mode andin a low conversion gain mode. If gain select transistor 108 isdisabled, pixel 34 will be placed in a high conversion gain mode. Ifgain select transistor 108 is enabled, pixel 34 will be placed in a lowconversion gain mode. When gain select transistor 108 is turned on, thedual conversion gain capacitor 110 may be switched into use to providefloating diffusion region 118 with additional capacitance. This resultsin lower conversion gain for pixel 34. When gain select transistor 108is turned off, the additional loading of the capacitor is removed andthe pixel reverts to a relatively higher pixel conversion gainconfiguration.

To allow overflow of charge from photodiode 102 to storage capacitor 110and increase dynamic range, control signal TH1 for threshold transistor105 may be set to an intermediate voltage level during the integrationtime. When the charge levels in photodiode 102 exceed a given level (setby control signal TH1), charge may overflow into capacitor 110. Forexample, the overflow charge may follow path 132. This example is merelyillustrative, and control signal TX for transfer transistor 104 mayinstead be set to an intermediate voltage level that allows charge topass to capacitor 110 when the charge levels exceed a given level in thephotodiode.

Including storage capacitor 110 (Cx) in the imaging pixel increases thedynamic range of the pixel relative to embodiments where the storagecapacitor is not included. However, the dynamic range may still belimited by the size of storage capacitor 110. For additionalimprovements to dynamic range, an arrangement of the type shown in FIG.4 may be used.

FIG. 4 is a circuit diagram of an illustrative imaging pixel with twocharge overflow paths to further extend the dynamic range of the imagingpixel. As shown in FIG. 4, much of the pixel circuit is the same asshown in FIG. 3. Transfer transistor 104 is included between photodiode102 and floating diffusion region 118. A reset transistor 106 is coupledbetween floating diffusion region 118 and voltage supply 124. Thefloating diffusion region is coupled to the gate of source followertransistor 112. Source follower transistor 112 is coupled betweenvoltage supply 120 and row select transistor 114. Row select transistor114 is coupled to a column output line. In this pixel, two column outputlines are included, so the column output line coupled to row selecttransistor 114 is referred to as column output line 116-1.

Similar to as in FIG. 3, a transistor 105 is coupled to photodiode 102between the photodiode and a storage capacitor 110 in FIG. 4. During theintegration time of pixel 34, control signal TH1 for transistor 105 maybe set to an intermediate voltage level. When the charge levels inphotodiode 102 exceed a given level (set by control signal TH1), chargemay overflow into capacitor 110. For example, the overflow charge mayfollow path 132.

In high incident light level conditions, the dynamic range of theimaging pixel may be limited by the storage capacity of storagecapacitor 110 (if storage capacitor provides the only overflow chargestorage as in FIG. 3). To increase the dynamic range, the pixel of FIG.4 includes an additional storage capacitor and an additional chargeoverflow path to the additional storage capacitor. A coupled gatestructure 152 (sometimes referred to as coupled gate 152, chargedirecting circuitry 152, etc.) may be used to selectively directoverflow charge to the additional storage capacitor.

Coupled gate 152 includes a first transistor 154 and second transistor156 coupled in series (to storage capacitor 162). Coupled gate 152 alsoincludes a reset transistor 158 that is coupled to a node 155 betweentransistors 154 and 156. Reset transistor 158 is coupled between node155 and bias voltage supply terminal 160 (that supplies a voltage Vyy).

Storage capacitor 162 is coupled to the gate of an additional sourcefollower transistor 164. Source follower transistor 164 may be coupledbetween voltage supply 166 (that supplies a voltage VDD) and row selecttransistor 168. Row select transistor 168 may be coupled to anadditional column output line 116-2. When row select transistor 168 isasserted, a voltage Vout2 indicative of the amount of charge on storagecapacitor 162 may be sampled onto column output line 116-2.

A gate terminal of threshold transistor 154 receives control signal TH2.A gate terminal of transistor 156 receives control signal INT. A gateterminal of transistor 158 receives control signal OF_RST. A gateterminal of transistor 168 receives control signal RS2. Transistor 156may sometimes be referred to as an integration transistor and transistor158 may sometimes be referred to as an overflow reset transistor.

During the integration time of pixel 34, control signal TH1 fortransistor 105 and control signal TH2 for transistor 154 may both be setto respective intermediate voltage levels. When the charge levels inphotodiode 102 exceed a first given level (set by control signal TH1),charge may overflow into capacitor 110 (following path 132). When thecharge level (e.g., in photodiode 102 and storage capacitor 110) exceedsa second given level (for which the channel barrier of TH2 is lower thanthe channel barrier of TH1), charge may overflow through transistor 154to node 155 (following path 170).

During the integration period, transistors 156 and 158 may be assertedin a mutually exclusive manner (or nearly mutually exclusive manner) todirect the overflow charge along one of two paths. Transistors 156 and158 may be in the same state (e.g., both asserted or both deasserted)for less than 20% of the integration time, less than 10% of theintegration time, less than 5% of the integration time, less than 2% ofthe integration time, less than 1% of the integration time, less than0.1% of the integration time, less than 0.01% of the integration time,etc. If transistor 158 is asserted, transistor 156 will be deasserted.In this state, charge will be drained to voltage supply 160. Iftransistor 156 is asserted, transistor 158 will be deasserted. In thisstate, charge will be transferred to storage capacitor 162 (Cy). Once inthe storage capacitor 162, the charge may be sampled by using sourcefollower transistor 164, row select transistor 168, and column outputline 116-2 (sometimes referred to as a readout circuit or readoutcircuitry).

The arrangement of coupled gate 152 may allow the dynamic range of thepixel to be easily controlled by adjusting the amount of timetransistors 156 and 158 are each asserted. For example, consider thetiming diagram of FIG. 5. FIG. 5 shows the OF_RST control signal and INTcontrol signal during the integration period of pixel 34. As shown, whenOF_RST is high, INT is low (and vice-versa). The signals may cyclebetween high and low states at consistent rate. The amount of time theOF_RST signal is high in each cycle sets the dynamic range of the pixel.For example, in FIG. 5 OF_RST is high for 75% of each cycle. For theremaining 25% of the cycle, OF_RST is low (and INT is high). This meansthat, of the overflow charge received by coupled gate structure 152, 75%is drained to voltage supply 160 and only 25% is transferred to storagecapacitor Cy. This known ratio may be used to extrapolate a detectedoverflow signal. In this case, for example, the amount of charge sampledfrom Cy may be multiplied by four (because integration was onlyoccurring 25% of the time) to determine how much overflow charge wasreceived by coupled gate 152. This effectively increases the storagecapacity of storage capacitor 162 by four times, which improves thedynamic range of the pixel.

The frequency of asserting between OF_RST control signal and INT controlsignal can be controlled in a psuedo random nature in order to mitigateLED frequency gaps which could be present with a constant frequency. Forexample, if the OF_RST is configured to be asserted for 50% of theintegration time (and the RST control signal is therefore alsoconfigured to be asserted for 50% of the integration time), the OF_RSTsignal could be asserted with a constant frequency (e.g., 1 millisecondasserted, then 1 millisecond deasserted, then 1 millisecond asserted,then 1 millisecond deasserted, etc.). Alternatively, the duration ofeach assertion and deassertion may vary across the integration timewhile still totaling 50% of the integration time.

The dynamic range of the pixel may therefore be easily controlled byselecting the amount of time OF_RST is asserted. The more time OF_RST isasserted, the higher the dynamic range of the pixel will be. However,the more time OF_RST is asserted, the worse the signal to noise ratio(SNR) of the sample will be. In FIG. 6, an alternate timing diagram forthe OF_RST control signal and INT control signal is shown during theintegration period of pixel 34. As shown, when OF_RST is high, INT islow (and vice-versa). The signals may cycle between high and low statesat a consistent rate. In FIG. 6, OF_RST is high for 25% of each cycle.For the remaining 75% of the cycle, OF_RST is low (and INT is high).This means that, of the overflow charge received by coupled gatestructure 152, 25% is drained to voltage supply 160 and 75% istransferred to storage capacitor Cy. When using the duty cycle of FIG.6, the pixel will have a lower dynamic range compared to the duty cycleof FIG. 5. However, the pixel will have an improved signal to noiseratio in FIG. 6 compared to FIG. 5.

Reset transistor 158 may be asserted for any desired percentage of timeduring the integration time of the pixel. The duty cycle may bepredetermined or may be updated dynamically by control circuitry in theimage sensor (e.g., in response to incident light levels). For example,reset transistor 158 may be asserted more than 95% of the time, morethan 90% of the time, more than 75% of the time, more than 65% of thetime, more than 50% of the time, more than 40% of the time, more than30% of the time, more than 20% of the time, more than 10% of the time,more than 5% of the time, less than 95% of the time, less than 90% ofthe time, less than 75% of the time, less than 65% of the time, lessthan 50% of the time, less than 40% of the time, less than 30% of thetime, less than 20% of the time, less than 10% of the time, less than 5%of the time, between 60% and 90% of the time, between 10% and 40% of thetime, etc. Said another way, the time that reset transistor 158 isasserted divided by the time that reset transistor 158 is not asserted(and integration transistor 156 is asserted) may be equal to 1, greaterthan 1, greater than 2, greater than 3, greater than 4, greater than 10,greater than 20, less than 20, less than 10, less than 0.5, less than0.1, less than 0.05, more than 0.05, etc.

At the end of the integration time, charge may be present in photodiode102, storage capacitor 110, and/or storage capacitor 162. Charge fromall three locations may therefore be sampled during the readout process.To read out the charge from storage capacitor 162, row select transistor168 may be asserted (sampling the voltage from storage capacitor 162onto column output line 116-2). This sampling may occur while transistor156 is deasserted. Then, to reset capacitor Cy, transistors 156 and 158may be simultaneously asserted. Once storage capacitor 162 is reset,charge may be transferred from storage capacitor 110 to storagecapacitor 162 by asserting transistors 154 and 156. The charge fromstorage capacitor 162 (which now has the charge originally stored instorage capacitor 110) may then again be sampled by asserting row selecttransistor 168.

To improve signal-to-noise ratio in the samples, double sampling may beperformed. For example, after the charge in storage capacitor 162 issampled for the first time and storage capacitor 162 is reset, the resetcharge level of the storage capacitor may be sampled (e.g., by assertingrow select transistor 168). This reset charge level may be subtractedfrom the first sample of storage capacitor Cy to account for the resetcharge level. Then, the charge from storage capacitor 110 is transferredto storage capacitor 162 and sampled. The previously obtained resetcharge level may be subtracted from this second sample to correct forthe reset charge level. Alternatively, once the second sample isobtained from storage capacitor 162, the storage capacitor 162 may beagain reset and a second reset charge level sample may be obtained. Thesecond reset charge level may optionally be used to correct the secondsample.

Also during readout, transfer transistor 104 may be asserted to transfercharge from photodiode 102 to floating diffusion region 118. This chargemay then be sampled onto column output line 116-1 by asserting rowselect transistor 114. Correlated double sampling may be performedduring this sampling.

If desired, photodiode 102 may be reset by asserting transistors 105,154, and 158.

Herein, transistors 105 and 154 are used as examples of a structure witha potential barrier that allows charge to overflow the structure whencertain charge levels are exceeded. This example is merely illustrative.Other structures may be used to establish this potential barrier ifdesired.

Additionally, the examples of storage capacitors 110 and 162 for storingcharge are merely illustrative. Any desired charge storage region (e.g.,a storage diode, storage gate, etc.) may be used in place of storagecapacitor 110 and/or storage capacitor 162 if desired.

FIG. 7 is a flowchart showing illustrative method steps for operating animaging pixel of the type shown in FIG. 4. At step 202, the pixel mayhave an integration time. During the integration time, transfertransistor 104 is deasserted, transistors 105 and 154 are set tointermediate levels, and transistors 156 and 158 are alternatelyasserted and deasserted. During the integration time, photodiode 102will generate charge in response to incident light. Initially, thecharge will accumulate in the photodiode. Once the accumulated chargeexceeds a first level (set by TH1), the charge will overflow throughtransistor 105 into storage capacitor 110 (along path 132 in FIG. 4). Ifenough charge is generated, the amount of charge in storage capacitor110 will exceed a second level (set by TH2) and charge will overflowthrough transistor 154 and pass to node 155 (along path 170 in FIG. 4).At node 155, the charge will either be drained to voltage supply 160(when transistor 158 is asserted) or integrated and stored at storagecapacitor Cy (when transistor 156 is asserted).

At the end of the integration time, the amount of charge at storagecapacitor Cy may be sampled in step 204. Transistor 168 may be asserted(while transistor 156 is deasserted) to sample the charge. Aftersampling the amount of charge at storage capacitor Cy, storage capacitorCy may be reset at step 206. To reset storage capacitor Cy, transistors156 and 158 may be simultaneously asserted. Next at step 208, the resetlevel of storage capacitor Cy may be sampled by asserting transistor168. After sampling the reset level, charge may be transferred fromstorage capacitor Cx to storage capacitor Cy at step 210. Transistors154 and 156 may be asserted to transfer the charge from storagecapacitor Cx to storage capacitor Cy. The charge in storage capacitor Cymay then again be sampled at step 212.

Before, during, or after steps 204-212 (e.g., at some point after theintegration time is complete), the charge in photodiode 102 may also besampled. As one example, the floating diffusion region may be reset andthe reset charge level of the floating diffusion region may be sampled(by asserting transistor 114). Transfer transistor 104 may then beasserted to transfer charge to the floating diffusion region. Thefloating diffusion region may then again be sampled.

The steps shown in FIG. 7 are merely illustrative. It should beunderstood that modifications may be made to these steps as desired. Forexample, an additional reset sample of storage capacitor Cy may beobtained after step 212 as previously discussed.

The pixel circuit shown in FIG. 4 is merely illustrative. Otherarrangements may be possible that use a coupled gate structure forvariable dynamic range improvements. FIG. 8 is a diagram of a pixel 34that is similar to the pixel of FIG. 4. However, in FIG. 8, pixel 34includes an additional transistor 108 coupled between storage capacitor110 and floating diffusion region 118. Transistor 108 may optionally beused to read out charge from storage capacitor Cx using source followingtransistor 112. Alternatively, charge from storage capacitor Cx may betransferred to storage capacitor Cy and sampled from storage capacitorCy as previously discussed.

Additionally, FIG. 8 shows how pixel 34 may be split between multiplesubstrates. In some embodiments, the components of pixel 34 shown inFIG. 8 may be formed in a single substrate (e.g., a semiconductorsubstrate). To allow more room for photodiode 102 and/or decrease thesize of the pixels, pixels 34 may optionally be split between two ormore substrates. FIG. 8 shows an illustrative embodiment where a firstplurality of the pixel components is formed in substrate 182 (sometimesreferred to as semiconductor substrate 182, wafer 182, upper substrate182, etc.) and a second plurality of the pixel components is formed insubstrate 184 (sometimes referred to as semiconductor substrate 184,wafer 184, lower substrate 184, etc.). The first and second substratesmay be connected by conductive interconnect layer 186. Interconnectlayer 186 may include one or more of a conductive pad, a micro-pad, aconductive interconnect structure, or a conductive via that connects thetwo substrates. A respective interconnect layer 186 may be includedbetween the two substrates for every pixel in the pixel array.

In FIG. 8, conductive interconnect layer 186 is formed between thresholdtransistor 154 and threshold transistor 105 (with storage capacitor 110being formed in substrate 182). This example is merely illustrative. Ingeneral, a conductive interconnect layer between the two substrates maybe formed at any desired point(s) within the pixel circuit. Two or moreinterconnect layers may be formed between the first and second substratewithin a single pixel. For example, an additional interconnect layer mayalso be included between floating diffusion region 118 and sourcefollower transistor 112.

In yet another alternative, pixel 34 may be split between three (ormore) substrates. For example, transistors 168 and 164 as well asstorage capacitor 162 may instead be formed in a third substrate. Anadditional interconnect layer may couple the third substrate to thesecond substrate between storage capacitor 162 and transistor 156.

The aforementioned examples are merely illustrative. In general, pixel34 may be split between any desired number of substrates in any desiredmanner.

FIG. 9 is a diagram of pixels that share a floating diffusion region. Asshown in FIG. 9, pixel 34-1 includes all of the components of the pixelshown in FIG. 4. Pixel 34-2, however, does not include its own resettransistor 106, source follower transistor 112, or row select transistor114. Instead, the transfer transistor of pixel 34-2 is coupled tofloating diffusion 118 in pixel 34-1. In other words, the floatingdiffusion region 118 and corresponding readout circuitry are sharedbetween pixels 34-1 and 34-2. Any desired number of pixels (in anydesired locations) may share a floating diffusion region andcorresponding readout circuitry.

FIGS. 10 and 11 show illustrative sharing arrangements for pixels in thepixel array. In FIG. 10, 1×2 groups 192 of pixels (with 2 adjacentpixels in a single column and two rows) each share a single floatingdiffusion region. As shown in FIG. 10, pixels 34-1 and 34-2 sharefloating diffusion region FD (and corresponding readout circuitry). FIG.11 shows an alternate arrangement where 2×2 groups 194 of pixels (with 4adjacent pixels in two columns and two rows) each share a singlefloating diffusion region. As shown in FIG. 11, pixels 34-1, 34-2, 34-3,and 34-4 share a floating diffusion region FD (and corresponding readoutcircuitry). These arrangements are merely illustrative. Any desirednumber of pixels (e.g., two, three, four, more than four, nine, morethan nine, etc.) may share a single floating diffusion region. Thepixels may be horizontally adjacent (e.g., in the same row), may bevertically adjacent (e.g., in the same column), may be diagonallyadjacent (e.g., as pixels 34-1 and 34-4 in FIG. 11), or may not beadjacent (e.g., with an intervening pixel that does not share thefloating diffusion region).

When two or more pixels share a floating diffusion region, the chargefrom the two pixels may be readout individually or cumulatively. Forexample, charge from two or more pixels may be transferred to thefloating diffusion region (e.g., binned) and read out as a singlesignal. Alternatively, charge from a first pixel may be read out usingthe floating diffusion region. Then, charge from a second pixel may besubsequently read out using the floating diffusion region. This allowsthe sensor to operate in either a binning mode (with lower resolutionbut higher light sensitivity) or a normal mode (with higher resolution).

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the art. Theforegoing embodiments may be implemented individually or in anycombination.

What is claimed is:
 1. An imaging pixel comprising: a photodiodeconfigured to generate charge in response to incident light; a floatingdiffusion region; a transfer transistor coupled between the photodiodeand the floating diffusion region; a first charge storage region,wherein a first portion of the charge from the photodiode is configuredto overflow to the first charge storage region; and a second chargestorage region, wherein a second portion of the charge from thephotodiode is configured to overflow to the second charge storageregion.
 2. The imaging pixel defined in claim 1, further comprising:charge directing circuitry interposed between the photodiode and thesecond charge storage region.
 3. The imaging pixel defined in claim 2,wherein the charge directing circuitry comprises a first transistor, asecond transistor that is coupled between the first transistor and thesecond charge storage region, and a third transistor that is coupledbetween the first transistor and a voltage supply.
 4. The imaging pixeldefined in claim 3, further comprising: a fourth transistor coupledbetween the first transistor and the photodiode.
 5. The imaging pixeldefined in claim 4, wherein the fourth transistor is coupled between thephotodiode and the first charge storage region.
 6. The imaging pixeldefined in claim 1, further comprising: a first source followertransistor coupled to the floating diffusion region; and a second sourcefollower transistor coupled to the second charge storage region.
 7. Theimaging pixel defined in claim 1, wherein the first charge storageregion is a first storage capacitor and the second charge storage regionis a second storage capacitor.
 8. The imaging pixel defined in claim 1,wherein the photodiode and the transfer transistor are formed in a firstsubstrate, wherein the second charge storage region is formed in asecond substrate, and wherein the imaging pixel further comprises aconductive interconnect structure between the first and secondsubstrates.
 9. An image sensor comprising an array of imaging pixels,wherein each imaging pixel comprises: a photodiode; a first storagecapacitor; a first transistor interposed between the photodiode and thefirst storage capacitor; a second storage capacitor; second and thirdtransistors coupled in series between the second storage capacitor andthe first transistor; a bias voltage supply terminal; and a fourthtransistor coupled between the bias voltage supply terminal and a nodethat is interposed between the second and third transistors.
 10. Theimage sensor defined in claim 9, wherein each imaging pixel furthercomprises: a floating diffusion region; and a transfer transistorinterposed between the floating diffusion region and the photodiode. 11.The image sensor defined in claim 10, wherein each imaging pixel furthercomprises: a first source follower transistor, wherein the floatingdiffusion region is coupled to a gate of the first source followertransistor; a first column output line; and a first row selecttransistor interposed between the first source follower transistor andthe first column output line.
 12. The image sensor defined in claim 11,wherein each imaging pixel further comprises: a second source followertransistor, wherein the second storage capacitor is coupled to a gate ofthe second source follower transistor; a second column output line; anda second row select transistor interposed between the second sourcefollower transistor and the second column output line.
 13. The imagesensor defined in claim 12, wherein each imaging pixel furthercomprises: a fifth transistor that is interposed between the firststorage capacitor and the floating diffusion region.
 14. The imagesensor defined in claim 9, wherein the image sensor comprises first andsecond substrates, wherein the photodiode and first transistor of eachimaging pixel are formed in the first substrate, wherein the second andthird transistors of each imaging pixel are formed in the secondsubstrate.
 15. The image sensor defined in claim 14, wherein eachimaging pixel comprises: a conductive interconnect structure between thefirst transistor in the first substrate and the second transistor in thesecond substrate.
 16. A method of operating an imaging pixel thatincludes a photodiode, a floating diffusion region, first and secondcharge storage regions, a first transistor interposed between thephotodiode and the first charge storage region, and a second transistorinterposed between the first transistor and the second charge storageregion, the method comprising: with the photodiode, generating charge inresponse to incident light during an integration time; setting the firsttransistor to a first intermediate voltage during the integration time,wherein a first amount of the charge that exceeds a first charge levelpasses through the first transistor into the first charge storageregion; and setting the second transistor to a second intermediatevoltage during the integration time, wherein a second amount of thecharge that exceeds a second charge level that is greater than the firstcharge level passes through the second transistor.
 17. The methoddefined in claim 16, wherein the second amount of charge is alternatelytransferred to the second charge storage region and drained to a biasvoltage supply terminal.
 18. The method defined in claim 16, furthercomprising: after the integration time is complete, reading out a firstsample from the second charge storage region; after reading out thefirst sample from the second charge storage region, resetting the secondcharge storage region; and after resetting the second charge storageregion, transferring charge from the first charge storage region to thesecond charge storage region.
 19. The method defined in claim 18,further comprising: after resetting the second charge storage region andbefore transferring charge from the first charge storage region to thesecond charge storage region, reading out a second sample from thesecond charge storage region; and after transferring charge from thefirst charge storage region to the second charge storage region, readingout a third sample from the second charge storage region.
 20. The methoddefined in claim 16, wherein the imaging pixel further comprises a thirdtransistor that is coupled between the second transistor and a biasvoltage supply terminal and a fourth transistor that is coupled betweenthe second transistor and the second charge storage region, the methodfurther comprising: alternately asserting the third and fourthtransistors during the integration time.